1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, the present invention relates to a secondary electrochemical cell dischargeable under both a constant discharge rate and a pulse discharge rate. Cardiac defibrillators present both electrical power requirements.
The constant discharge rate portion of the secondary cell of the present invention, referred to hereinafter as the medium rate region, preferably includes a high mass, low surface area lithium-retention cathode structure associated with a carbonaceous anode electrode in a side-by-side prismatic configuration. The pulse discharge rate portion of the secondary cell of the present invention, referred to hereinafter as the high rate region, preferably includes a high surface area lithium-retention cathode associated with a carbonaceous anode in a side-by-side prismatic configuration. Preferably the same anode structure is electrically associated with both the medium rate lithium-retention cathode region and the high rate lithium-retention cathode region housed within the same hermetically sealed casing. This structure defines what is meant by a medium rate region and a high rate region contained within the same secondary electrochemical cell.
2. Prior Art
Traditionally, cardiac defibrillator cells have been built using a multiplate electrode design. The cell designer must decide between providing additional electrochemically active components for increased mass and energy density or providing increased surface area for greater power density. Because of the wide disparity in the energy/power requirements placed upon a cardiac defibrillator cell or battery, that being intermittent low rate and high rate operation, a compromise is often decided upon. However, any design attempt to balance the energy/power requirements placed upon the cell or battery by the defibrillator device must not consequently produce unwanted self-discharge reactions. This compromise can provide for inefficiency and can decrease the overall gravimetric and volumetric energy density of the cell.
It is generally accepted that when low electrical currents are desired, the electrodes within a cell, whether of a primary or a secondary configuration, should have as much mass and as little surface area as possible. At the expense of power density, this provides for increased energy density while the low electrode surface area minimizes undesirable self-discharge reactions. Conversely, when larger electrical discharge currents are required, electrode surface area and power density are maximized at the expense of energy density and self-discharge rate.
The secondary cell of the present invention having an electrode assembly with differing discharge rate portions fulfills this need. The present secondary cell comprises regions containing a low interelectrode surface area in a side-by-side, prismatic configuration, preferred for routine monitoring by a device, for example a cardiac defibrillator, and regions containing a high interelectrode surface area in a side-by-side, prismatic configuration for use when high rate electrical pulse charging of capacitors is required with minimal polarization. It is believed that the present secondary electrochemical cell having electrodes with differing discharge rate regions represents a pioneering advancement wherein a medium discharge rate region and a high discharge rate region are provided within the same case for the purpose of having the cell supply at least two different electrical energy requirements.
The present invention provides an improved multiplate electrode design for an electrochemical lithium ion secondary cell dischargeable to provide background current intermittently interrupted or supplemented by current pulse discharge. The disclosed secondary cell is of a case-negative design in which the carbonaceous anode assembly is in electrical contact with the case. Two positive terminal pins are respectively connected to two independent lithium-retention cathode regions. One lithium-retention cathode region has a relatively low surface area and high density for providing low electrical current on the order of microamperes to milliamperes and the other lithium-retention cathode region has a relatively high surface area for providing high electrical current on the order of several hundred milliamperes to amperes.
The medium rate, constant discharge region of the present secondary cell comprises a lithium-retention cathode structure of one or more cathode plates flanked on either side by a carbonaceous anode. The lithium-retention cathode material, which preferably comprises an air stable lithiated compound, suitable conductive additive(s) and a binder, may be in a dry powder form and is pressed onto a conductive metal screen or foil. The carbonaceous anode preferably consists of carbon fibers, mesocarbon microbeads, graphitic carbon, non-graphitic carbon, petroleum coke, and other types of carbon that are also pressed onto a conductive metal screen or foil. A metallic lead connects the medium rate cathode region to a feedthrough terminal pin in the battery header which is insulated from the battery case by a glass-to-metal seal. The anode is either connected to the case resulting in a case-negative configuration or to another feedthrough pin also located in the header of the battery. A separator prevents short circuiting between the couple.
The high rate, pulse discharge region of the present secondary cell comprises a lithium-retention cathode structure of one or more cathode plates flanked on either side by the same anode that is coupled to the medium rate region. The interelectrode surface area of the high rate region is greater than that of the medium rate region to deliver high current pulses during device activation. Preferably the medium rate region contributes greater than 10% of the total energy density provided by the cell while having less than 50% of the total cathode surface area. Still more preferably, the medium rate region contributes greater than 10% of the total energy density provided by the cell while having less than 30% of the total cathode surface area.
Thus, the present invention offers the advantage of having both a medium rate, constant discharge or constant drain region and a high rate, pulse discharge region provided within the same secondary electrochemical cell. The electrochemical couple used for both the medium rate region and the high rate region is, for example, a carbon/lithiated oxide couple such as a carbon/LiCoO2 couple. However, both discharge region couples need not necessarily be identical. Secondary electrochemical cells according to the present invention having medium rate and high rate discharge regions can be constructed and designed to meet the drain rate and current discharge requirements of a particular application.
These and other aspects of the present invention will become more apparent to those skilled in the art by reference to the following description and the appended drawings.